1. Field of the Invention
The invention relates to a belt tightening drive for tightening a safety belt which can be wound up around a winding shaft.
Said type of reversible belt tightening drive is disclosed for example in DE 10 2008 048 339 A1, in which an electric motor is driving a spindle shaft by way of a first gear designed as a spiral gear, said spindle shaft in turn driving an output wheel by way of a second gear, said output wheel being arranged torsionally rigid on a winding shaft for winding up a safety belt. Said second gear is formed by a worm drive arranged on the spindle shaft and an external helical gear of the output wheel.
A reversible belt tightener is activated when necessary, i.e. for example in connection with a minor crash or an impending crash, during off-road journeys and in connection with braking deceleration of the vehicle which brings about the tightening of the safety belt. To maintain the belt tightening in these kinds of situations, a high electric current is used to energize the electric motor which causes an undesirably high strain on the vehicle's supply system.
In the event of a crash, the belt tightener is activated pyrotechnically and represents a full-load tightening resulting in the decoupling of the belt tightener from the electric motor. As a result of the subsequent holding of the safety belt by means of the electrical drive with the residual energy of the electric motor, it is re-coupled onto the belt tightener which can potentially cause impairments in the power limit.
Moreover, said residual energy of the electric motor causes a strain on the vehicle's supply system.
Furthermore, highly dynamic load statuses develop in connection with full load tightening. They result in high turning moments (holding moments) in the output stage of the belt tightener and hence in high gear tooth forces, in particular also in high axial forces in connection with the helical gears of the used gear components, said axial forces causing a deformation of the gear components such as the wheel body and the bearings in the housing of the belt tightener. This means a loss of engagement depth and causes axial relative shifts in the engagement of the gear components, as a consequence of which the active gear width is reduced considerably. The two negative effects result in a substantial reduction of the carrying capacity of the gear components. Moreover, high radial forces develop due to the engagement angle, resulting in shifts and deformations in consequence of which the engagement depth (active depth of teeth) and hence the coverage is reduced. This means a loss in carrying capacity and ultimately leads to damage and mechanical failure of the gearing components.